Confirmation of Gold Active Sites on Titanium-Silicalite-1-Supported

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Confirmation of Gold Active Sites on Titanium Silicalite-1 Supported Nano-Gold Catalysts for Gas-Phase Epoxidation of Propylene Zhishan Li, Jihai Zhang, Dongyu Wang, Weihua Ma, and Qin Zhong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08293 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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The Journal of Physical Chemistry

Confirmation of Gold Active Sites on Titanium Silicalite-1 Supported Nano-Gold Catalysts for Gas-Phase Epoxidation of Propylene Zhishan Li, Jihai Zhang, Dongyu Wang, Weihua Ma* and Qin Zhong* School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, People’s Republic of China Correspondence to: Weihua Ma (E-mail: [email protected]) Qin Zhong (E-mail: [email protected]) ABSTRACT: The nature of gold active sites on Au-Ti catalysts dictates the reactivity and selectivity during propylene epoxidation reactions. The type of the sites and the location of Au clusters on the external surfaces of TS-1 or inside the TS-1 nanopores have remained the topic of interest. We synthesized and characterized dispersed Au clusters on titanium silicate-1 (Au/TS-1), uncalcined TS-1 (Au/un TS-1) and TS-1 coated with silicalite-1 (Au/S-1/TS-1), to probe the nature of gold active sites for gas-phase epoxidation of propylene. The results of Au/TS-1 and Au/S-1/TS-1 show the importance of Au nanoclusters (< 1.0 nm) inside TS-1 nanopores. Propylene oxide (PO) rate of Au/un TS-1 suggests Au nanoclusters (< 1.0 nm) on the external surfaces also play an important role. Based on this conclusion, better performance for direct propylene epoxidation can be developed over Au-Ti catalysts with Au nanoclusters (< 1.0 nm) either inside TS-1 nanopores or on the external surfaces. 1 Introduction Propylene oxide (PO) is an important chemical intermediate and precursor for the

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production of polyether polyols, propylene glycol, etc.1 Industrially, PO is manufactured by either chlorohydrin or co-production routes.2 The former process consumes a large amount of chlorine and causes environmental pollution. The latter process is capital intensive and produces equimolar amounts of co-products.3 Since Haruta et al. first reported the activity of dispersed Au nanoparticles on anatase TiO2 in the direct gas-phase epoxidation of propylene,4 the Au-Ti catalyst system has attracted significant interest. Direct propylene epoxidation with H2 and O2 is an environmentally friendly process, because H2O is the only by-product.5 It is generally accepted that the catalytic role of Au sites is to generate hydrogen peroxide (H2O2) and that of Ti sites is to epoxidize propylene in the Au-Ti system.6-8 Among various Ti-containing supports such as TiO2, TiO2/SiO2,9 Ti-TUD,10,

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TS-1,12-14 TS-β,15 Ti-MCM-41,16 Ti-MCM-4817 and Ti-SBA-1518, 19 (from metallic oxides, metal composite oxides to mesoporous and microporous zeolite), TS-1 has been shown to be a promising support for the higher dispersion of titanium isolation.20, 21

The isolated tetrahedral framework Ti species is vital to propylene epoxidation

reaction because Ti-O-Ti entities have been found to be detrimental for PO rate and to promote total combustion of propylene to CO2.20, 22-24 TS-1 zeolite material is of great significance to organic syntheses and much work so far has been focused on improving the property of TS-1.25-28 Besides propylene epoxidation, TS-1 also is applied in the epoxidations of allylic compounds such as allyl alcohol, allyl chloride, methallyl alcohol, crotyl alcohol and 1-butene-3-ol,29-32 and oxidation of alkanes,33 phenol hydroxylation34 and other kinds of reactions.35, 36

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In the field of gas-phase epoxidation of propylene, although Au/TS-1 shows good catalytic performance,12, 20 the specific gold active sites on Au/TS-1 catalysts and the appropriate Au cluster diameter are still under debate.37 It is meaningful to solve the problems and further improve the catalytic performance. A few groups have discussed the gold active sites for gas-phase epoxidation of propylene. Haruta et al. proposed that Au clusters with the average diameters between 1.0 nm and 2.0 nm over the TS-1 external surfaces are the active sites and the size of ~1.4 nm seemed to be more dominant.22, 38 Although Feng et al. reported that PO rate of ~ 125 gPO h-1 kgcat-1 was detected at 200 °C over Au-Ti catalyst with Au particle size of 3.0-5.0 nm on the external surfaces, they contributed the high value to parts of Au clusters migrated into the in situ nanoporous channels with time on stream.26 Delgass et al. verified that Au nanoclusters (< 1.0 nm) inside the TS-1 nanoporous channels are more active by density functional theory calculation39 and from experimental aspects.20, 37, 40 These studies have not reached an unambiguous conclusion on the location of the Au nanoclusters that leads to the desired selectivities for direct propylene epoxidation. In this work, we prepared three types of catalysts (Au/TS-1, un TS-1 and Au/S-1/TS-1) to investigate the location of Au nanoclusters. Based on the linear relationship between increased Au loading and improved PO rate,25 the inconsistent relationship for Au/TS-1 and consistency for Au/S-1/TS-1 suggest that Au nanoclusters (< 1.0 nm) inside the TS-1 nanopores are the dominant gold active sites for propylene epoxidation. In addition, PO rate of Au/un TS-1 with template (tetrapropylammonium, TPA+) inside the TS-1 nanopores, was detected at the early

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stage, implying that smaller Au nanoclusters (< 1.0 nm) on the external surfaces are the dominant gold active sites. Therefore, both Au nanoclusters (< 1.0 nm) inside the TS-1 nanopores and on the external surfaces are important for PO reaction. 2 Experimental 2.1 Synthesis of supports TS-1 support was synthesized by the hydrothermal synthesis method according to previously established techniques.41 For a typical synthesis of TS-1, 14.7 mL tetrapropylammonium hydroxide (TPAOH, 2.0 M in H2O) was added dropwise into 35 mL deionized water under vigorous stirring at room temperature (RT). The mixture of 30 mL tetraethyl orthosilicate (TEOS, AR) and 0.46 mL tetrabutylorthotitanate (TBOT, AR) was followed by dropwise addition at RT. Then the solution was heated to 50 °C and stirred until the mixture became clear, and heated to 80 °C and stirred for 30 min. The resultant solution was transferred into Teflon autoclave and placed in an oven at 170 °C for 24 h. The resulting solid was obtained by centrifugation (4500 rpm for 30 min), washed once with approximately 100 mL deionized water and dried overnight at 25 °C in a vacuum oven. The resulting white powder was un TS-1 (with TPA+ template). Finally, un TS-1 was calcined at 550 °C for 16 h with a constant ramping rate ~ 2 °C min-1 to remove the organic template to obtain the TS-1 support. The preparation of S-1/TS-1 was similar to the literatures.37, 42 For the first S-1 coating, 2.3 g TS-1 with smaller size (separated by a high-speed centrifuge) was added into 75 mL of 0.1 mol L-1 hydrochloric acid solution and stirred at 50 °C for 44 h. The process of acid treatment was used to remove extra-framework Ti and reduce

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surface Ti content.42 Then the TS-1 suspension was centrifuged at 4500 rpm for 30 min and washed twice with approximately 200 mL deionized water. Afterwards, the TS-1 seed was suspended in 50 mL of 0.1 mol L-1 ammonia solution for 1 h at RT, and the base-treated TS-1 was obtained after centrifugation at 4500 rpm for 30 min. The next step was coating TS-1 with S-1. For the first S-1 coating, 1.54 mL TPAOH and 11.3 mL absolute ethanol (EtOH, AR) was added dropwise into the mixture of 12.8 mL TEOS and 66.3 mL deionized water at RT. The base-treated TS-1 seed was added into the solution and heated to 60 °C, and stirred for 1 h before being transferred into Teflon autoclave and placed in an oven at 100 °C for 20 h. After crystallization, the suspension was centrifuged at 4500 rpm for 20 min and washed twice. The separated solid was suspended in 50 mL of 0.27 mol L-1 NH4OH solution and stirred at RT overnight, then separated and washed once by centrifugation at 4500 rpm for 20 min, and suspended in 15 mL of 0.12 mol L-1 NH4OH solution. For the second S-1 coating, 1.02 mL TPAOH, 7.52 mL ethanol, and 1 mL ethanolamine was added dropwise into the mixture of 8.5 mL TEOS and 44.2 mL deionized water at RT. The suspension was added to the solution and stirred at RT for 30 min before being transferred into Teflon autoclave and placed in an oven at 100 °C for 8 h. The resulting solid was obtained by centrifugation (4500 rpm for 10 min), washed twice with approximately 100 mL deionized water and dried overnight at 25 °C in a vacuum oven. Finally, the solid was calcined at 550 °C for 16 h with a ramping rate of ~ 2 °C min-1 to obtain S-1/TS-1. 2.2 Au deposition Au/TS-1 catalyst was prepared by the deposition precipitation (DP) method,9, 43

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with Na2CO3 or Cs2CO340 as precipitation agent. Approximately 0.1 g chloroauric acid (HAuCl4·4H2O, AR) was dissolved in 20 mL deionized water and followed by the addition of approximately 1.0 g TS-1. The mixture was stirred for 30 min at RT. Then precipitation agent was added to target the final pH (6.0-8.0) of the slurry and stirred at RT for 4 h. The resulting solid was separated by centrifugation (4500 rpm for 15 min), washed once with approximately 50 mL deionized water and dried under vacuum at 25 °C. Here, Au/TS-1 with Na2CO3 as precipitation agent is denoted as Au/TS-1 Na and Au/TS-1 with Cs2CO3 as precipitation agent is denoted as Au/TS-1 Cs. Au/un TS-1 Na and Au/un TS-1 Cs were prepared by the same method. For Au/S-1/TS-1, the slurry mixing time was extended to 10 h (denoted as Au/S-1/TS-1 Na 10 h and Au/S-1/TS-1 Cs 10 h) or 20 h (denoted as Au/S-1/TS-1 Cs 20 h). 2.3 Characterization The bulk structure of TS-1 support was determined by X-Ray Diffraction (XRD; Beijing Purkinje General) with Cu Kα radiation (36 kV, 30 mA) in the 2θ interval from 5° to 60° with a scanning rate of 8° min-1. The local environment of the titanium in the support was evaluated by Ultraviolet-Visible (UV-vis, Evolution 220) with BaSO4 as the reference. Fourier transform infrared spectroscopy (FTIR) spectra of samples were recorded on a Nicolet IS10 with pure KBr as background. BET surface area and the pore volume were measured by N2 adsorption isotherm (Micromeritics; ASAP 2020) with samples outgassed at 200 °C for at least 8 h before each measurement. The morphology of support was taken from a field emission scanning electron microscope (FESEM; Hitachi S-4800). The size of catalyst was observed by

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transmission electron microscope (TEM; Philips Tecnai 12). The metal contents in the catalyst (Ti and Au) were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES; Varian ICP 720) after dissolving the catalyst with aqua regia and hydrofluoric acid.20 The surface Ti content was obtained by X-ray photoelectron spectroscopy (XPS; PHI Quantera II). 2.4 Catalytic testing The gas-phase epoxidation of propylene was tested in a microcatalytic fixed-bed reactor with an inner diameter of 8.0 mm, where 0.15 g Au-Ti catalyst (60-80 mesh size) was loaded. The reaction temperature was measured with a thermocouple located in the center of the catalyst bed. The reactant mixture consisted of propylene (C3H6, 99.5%): oxygen (O2, 99.999%): hydrogen (H2, 99.999%): nitrogen (N2, 99.999%) = 3.5: 3.5: 3.5: 24.5 mL min-1 (a space velocity of 14000 mL h-1 gcat-1) under atmospheric pressure, as shown in Figure S1. The catalyst was heated from RT to reaction temperature (200 °C) at a rate of 1 °C min-1, then reactants and products were analyzed using an on-line gas chromatograph (GC; SP-3420A) equipped with FID (KB-wax capillary column) and TCD (Chromsorb 102 packed column). The KB-wax capillary column (0.3 mm × 50 m) was used to detect C3H6, PO, acetaldehyde, propionaldehyde, acetone, acrolein and other oxygenates. The Chromsorb 102 packed column (3 mm × 3 m) was used to detect H2 and O2. Such performances as PO rate, PO selectivity, C3H6 conversion and H2 efficiency of catalyst were measured, which were calculated by the normalization method. Blank test indicated that no PO was generated in the blank reactor.

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3 Results 3.1 Catalysts characterizations Figure 1 (a) shows the XRD patterns of TS-1, un TS-1 and S-1/TS-1. It can be seen that the XRD patterns are consistent with a typical MFI structure (double ten-membered ring cross-channels).42 Additionally, there is no Au diffraction peaks in Figure S2 (a1) (b1) (c1), indicating that Au is highly dispersed on TS-1, un TS-1 and S-1/TS-1. From Figure 1 (b) and Figure S2 (a2) (b2) (c2), only a major absorption peak at around 220 nm in the UV-vis spectra is observed, which is characteristic of Ti in tetrahedral coordination.44, 45 The lack of absorption peaks in the 330 nm region indicates no extra-framework titanium.44, 46 Figure 1 (c) shows the FTIR spectra of three supports. As can be seen, the absorption peak at 960 cm-1 is ascribed to the stretching of Ti-O-Si bond, which proves the existence of framework titanium.26 For the curve of un TS-1, two extra bands at ~ 2900 cm-1 and ~ 1500 cm-1 are attributed to -CH2 and -CH3 group, as a result of the remained TPA+ template. Furthermore, N2 adsorption-desorption isotherms of three supports are shown in Figure 1 (d). It can be concluded that TS-1 and S-1/TS-1 are a typical microporous material, and un TS-1 has no micropores because the nanoporous channels are filled with TPA+ template. (Figure 1) The FESEM images of TS-1, un TS-1 and S-1/TS-1 are shown in Figure 2. From Figure 2 (a) and Figure 2 (b), TS-1 and un TS-1 has the same size, which is measured to ~ 240 nm. But the external surface of un TS-1 seem less smooth than that of TS-1. It might be due to the fact that TPA+ template existed over un TS-1, which is

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proved by the EDS analysis in Figure 2 (b). From Figure 2 (c), the size of S-1/TS-1 is ~ 300 nm and the morphology is similar to the literature.37 EDS analysis in Figure 2 (c) shows that little Ti content is detected and the further TEM observation can demonstrate the successful preparation of S-1 coating. (Figure 2) The apparent BET surface area, pore volume and the bulk Ti content of supports, as well as Au loadings of catalysts are summarized in Table 1. The BET surface areas of TS-1, un TS-1 and S-1/TS-1 are ~ 425 m2 g-1, ~ 15 m2 g-1 and ~ 406 m2 g-1, respectively. Due to TPA+ template insides the un TS-1, the BET surface area of un TS-1 is much lower than that of TS-1. The Au loadings of Au/TS-1 Na and Au/TS-1 Cs are determined to be ~ 0.058 wt% and ~ 0.314 wt%, those of Au/un TS-1 Na and Au/un TS-1 Cs are ~ 0.025 wt% and ~ 0.092 wt%. The Au loadings of Au/S-1/TS-1 Na 10 h, Au/S-1/TS-1 Cs 10 h and Au/S-1/TS-1 Cs 20 h are ~ 0.043 wt%, ~ 0.149 wt% and ~ 0.156 wt%, respectively. These results show that using Cs2CO3 instead of Na2CO3 as the precipitation agent during the deposition step leads to a three- or four-fold increase in Au loadings. Delgass et al. attributed the difference to a strong interaction between Cs and Au,40 which is proved by the Ti 2p photoelectron spectra of Au/TS-1 Na and Au/TS-1 Cs samples, as shown in Figure S3. (Table 1) 3.2 Catalytic performance of catalysts

Table 2 shows the performance of propylene epoxidation over Au/TS-1, Au/un TS-1 and Au/S-1/TS-1 spent at (200±1) °C for 4 h, respectively. The specific catalytic 9



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performance vs. time-on-stream of catalysts is shown in Figure S4. As can be seen, PO rate of Au/TS-1 Na is ~ 125 gPO h-1 kgcat-1 and that of Au/TS-1 Cs is ~ 330 gPO h-1 kgcat-1. PO rate of the latter is almost threefold as high as that of the former. The result is similar to the literature.40 PO rate of Au/un TS-1 is low at the early stage of epoxidation reaction and then increases gradually, reaching a stable value of ~ 30 gPO h-1 kgcat-1 (for Au/un TS-1 Na) and ~ 58 gPO h-1 kgcat-1 (for Au/un TS-1 Cs) after 30 h. Here, PO rate of Au/un TS-1 Na (~ 30 gPO h-1 kgcat-1) is lower than that reported in the literature (~ 125 gPO h-1 kgcat-1),26 owing to the different Au loadings (~ 0.025 wt% vs. ~ 0.12 wt%). PO rates of Au/S-1/TS-1 Na 10 h, Au/S-1/TS-1 Cs 10 h and Au/S-1/TS-1 Cs 20 h are determined to be ~ 45, ~ 122 and ~ 130 gPO h-1 kgcat-1, corresponding to ~ 94, ~ 255 and ~ 270 gPO h-1 kgTS-1-1, respectively. There was also a threefold increase in PO rate of Cs modified catalysts than that with Na2CO3 as the precipitation agent. For Au/S-1/TS-1 Na 10 h, PO rate of ~ 94 gPO h-1 kgTS-1-1 is a little lower than that reported by Delgass et al. (~ 140 gPO h-1 kgTS-1-1)37 because of different Au loadings (~ 0.043 wt% vs. ~ 0.055 wt%). Thus, using Cs2CO3 to deposit nano-gold on three different supports caused a significant increase in PO rate of all catalysts by increasing Au loading and stabilizing Au clusters due to the interaction between Cs and Au. (Table 2) 4 Discussions 4.1 Au/TS-1 Na and Au/TS-1 Cs Au/TS-1 catalysts with Na2CO3 and Cs2CO3 as the precipitation agent were

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prepared to confirm the importance of Au nanoclusters (< 1.0 nm) inside the TS-1 nanopores. Figure 3 shows the TEM images of fresh Au/TS-1. As can be seen, the average particle size of TS-1 is ~ 240 nm, which is in agreement with the result of FESEM. The Au particles are not observed on the fresh Au/TS-1 Na, while very few Au particles with the diameters of (2.0-3.0) nm are found on fresh Au/TS-1 Cs. Figure S5 (a) shows the HAADF image of fresh Au/TS-1 and Figure S5 (b) shows Au elemental mapping images of Au/TS-1 Cs, indicating that the Au species were dispersed well on TS-1. (Figure 3) From Table 3, Au content is detected to be less than 0.1 wt% on the surfaces of Au/TS-1 Na and Au/TS-1 Cs by XPS. Combined with the ICP data in Table 1 (where Au loading of Au/TS-1 Cs was ~ 0.314 wt% and that of Au/TS-1 Na was ~ 0.058 wt%), it can be concluded that the increased Au loading over Au/TS-1 Cs is mainly located inside the TS-1 nanoporous channels. And from Table 2, PO rate of Au/TS-1 Cs is almost three times as high as that of Au/TS-1 Na. Therefore, Au nanoclusters (< 1.0 nm) inside the TS-1 nanopores are the dominant gold active sites for gas-phase epoxidation of propylene. The HRTEM image of spent catalyst demonstrates that Au particles smaller than 1.0 nm still remained after PO reaction, as shown in Figure S6. Figure S7 shows the results of catalytic performance for reused catalyst, suggesting that Au-Ti catalyst has a good stability. (Table 3) However, the increase of PO rate was not consistent with the increase of Au loading

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(~ 3 vs. ~ 4), because a linear relationship between gold loading and PO rate (gPO h-1 kgcat-1)25 has been pointed out by Delgass et al. We explain this inconsistency by proposing that a large amount of Au increased in the nanoporous channels and the other small amount of Au increased on the external surfaces of TS-1, as shown in Figure 4. It is known that nano-gold agglomerates easily12 and Au particle size generally increases with Au loading.25 Thus, a higher Au loading on the external surfaces of Au/TS-1 Cs tends to severe agglomeration and larger Au particles are formed than that of Au/TS-1 Na, which corresponds to the conclusion that the catalyst with higher Au loading (> 0.1 wt%) might suffer sintering of the small gold clusters.40 Moreover, Oyama et al. pointed out that on the larger Au particles, the formed H2O2 cannot move to Ti sites because of the longer distance, and these hydrogen peroxide species decompose to H2O or are involved in nonselective oxidation,11 leading to the decrease of PO rate. If Au nanoclusters smaller than 1.0 nm deposited not only on the external surfaces of TS-1 but also inside the nanopores of TS-1 are the real active sites for PO reaction, this inconsistency would be explained clearly. The increase of Au loading inside the nanopores of Au/TS-1 Cs improved PO rate, but the increase of Au loading on the external surfaces of Au/TS-1 Cs has a negative effect, resulting in the fourfold increase of Au loading and threefold increase of PO rate for Au/TS-1 Cs than those of Au/TS-1 Na. (Figure 4) In addition, larger Au particle size decreases PO selectivity as well as H2 efficiency.11 From Table 2, PO selectivity and H2 efficiency of Au/TS-1 Cs are ~ 87%

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and ~ 10%, and those of Au/TS-1 Na are ~ 95% and ~ 33%. Although the increase of Au loading improved PO rate of Au/TS-1 Cs, larger Au particle formed on external surfaces of Au/TS-1 Cs decreased PO selectivity and H2 efficiency. Therefore, both the smaller Au particles (< 1.0 nm) inside the nanopores and on the external surfaces of TS-1 are more active for PO reaction. 4.2 Au/S-1/TS-1 Na and Au/S-1/TS-1 Cs To further explain the inconsistent relationship of Au/TS-1 catalysts between gold loading and PO rate, Au/S-1/TS-1 catalysts were prepared and tested. Figure 5 shows the TEM image of S-1/TS-1. It can be concluded that the preparation of coating TS-1 cores with S-1 shells was successful. The average particle size of smaller TS-1 is ~ 160 nm and that of S-1/TS-1 is ~ 300 nm. Because nano-gold has been suggested to preferentially anchor at Ti sites,25 Au clusters might prefer to be present inside the TS-1 nanopores compared to S-1 shells.37 It could be supposed that Au clusters were deposited inside the TS-1 nanopores and little amount was present on the external surfaces of TS-1. To some extent, Au loading is proportional to the amount of TS-1 and it is appropriate to describe PO rate per kilogram of TS-1 (gPO h-1 kgTS-1-1). (Figure 5) From Table 2, for different precipitation agents and the same mixing time, PO rate of Au/S-1/TS-1 Na 10 h is ~ 94 gPO h-1 kgTS-1-1 and that of Au/S-1/TS-1 Cs 10 h is ~ 255 gPO h-1 kgTS-1-1. There was nearly a threefold increase in PO rate of the latter than that of the former. Combined with Table 1, Au loading of Au/S-1/TS-1 Cs 10 h was also three times higher than that of Au/S-1/TS-1 Na 10 h. Here, the increase of PO

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rate was nearly consistent with the increase of Au loading (~ 3 vs. ~ 3). This consistency is due to the increase of Au nanoclusters in the TS-1 nanoporous channels, as shown in Figure 6. Furthermore, it implies that the explanation of inconsistency between PO rate and Au loading for Au/TS-1 Na and Au/TS-1 Cs (~ 3 vs. ~ 4) is reasonable. (Figure 6) For the same precipitation agent and different mixing time, PO rate of Au/S-1/TS-1 Cs 10 h is ~ 255 gPO h-1 kgTS-1-1 and that of Au/S-1/TS-1 Cs 20 h is ~ 270 gPO h-1 kgTS-1-1. As seen from Table 1 and Table 2, Au loading of Au/S-1/TS-1 Cs 10 h increases from ~ 0.149 wt% to ~ 0.156 wt% of Au/S-1/TS-1 Cs 20 h, and PO rate improves from ~ 255 to ~ 270 gPO h-1 kgTS-1-1, correspondingly. There is little difference between ~ 0.149% and ~ 0.156 wt% as mixing time extended. It can be considered that after 20 h of mixing time the balance of Au deposition was reached. Since PO only produces at Au-Ti sites,47 PO rate of Au/S-1/TS-1 Cs was almost as high as that of Au/TS-1 Cs (~ 270 vs. ~ 330 gPO h-1 kgTS-1-1), and it can be concluded that Au nanoclusters (< 1.0 nm) inside TS-1 nanoporous channels are the dominant gold active sites for propylene oxidation. 4.3 Au/un TS-1 Na and Au/un TS-1 Cs To prove the importance of Au nanoclusters on the external surfaces, Au/un TS-1 with TPA+ template inside the TS-1 nanopores was prepared. Because the TPA+ template on the internal surfaces is stable at 200 °C,26 and the amount of H2O2 produced at the early stage of reaction is not enough to decompose the organic

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template and form the in situ nanoporous channels, the PO rates vs. time-on-stream at the beginning for Au/un TS-1 spent at 200 °C were tested, as shown in Figure 7. (Figure 7) According to the result of BET apparent surface area for Au/un TS-1 spent at 200 °C for 1 h,20 there was no nanoporous channels formed without the removal of template. As seen from Figure 7, after the catalysts spent at 200 °C for 1 h, PO rate of Au/un TS-1 Na is ~ 1.3 gPO h-1 kgTS-1-1 and that of Au/un TS-1 Cs is ~ 2.0 gPO h-1 kgTS-1-1 (owing to the different Au loadings), implying that Au nanoclusters on the external surfaces of Au/un TS-1 catalysts also play an important role in direct propylene epoxidation. Figure 8 shows the diagram of this process. (Figure 8) Here, Au clusters smaller than 1.0 nm on the external surfaces of TS-1 are regarded as the main active sites. We demonstrated it by keeping Au/un TS-1 Cs at RT for long time (around six months) or calcining at 200 °C for several hours to obtain the larger Au sizes. Then the TEM and HRTEM images were observed, as shown in Figure S8. The catalytic performances were tested, as shown in Figure S9. As can be seen, Au particles are observed on the surfaces of un TS-1 and the size is mainly in the range of (1.0-10.0) nm. PO rates of treated Au/un TS-1 Cs are both lower than 1.0 gPO h-1 kgcat-1 at the initial stage of 1 h. Compared to fresh catalyst with PO rate of ~ 2.0 gPO h-1 kgcat-1, the obvious decrease of PO rate is due to the increase of greater Au particles (1.0-10.0 nm) and the corresponding decrease of smaller Au clusters (< 1.0 nm) on the surfaces of Au/un TS-1 Cs. Therefore, Au nanoclusters (< 1.0 nm) on the

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external surfaces are also the dominant active sites for PO reaction. 5 Conclusion Three supports (TS-1, S-1/TS-1 and un TS-1) were synthesized by the hydrothermal synthesis. Then nano-gold was deposited by the DP method using Na2CO3 or Cs2CO3 as the precipitation agent to obtain the catalysts (Au/TS-1, Au/TS-1 and Au/S-1/TS-1), which were used for gas-phase epoxidation of propylene. The linear relationships between Au loading and PO rate for Au/TS-1 (inconsistency, ~ 4 vs. ~ 3) and Au/S-1/TS-1 (consistency, ~ 3 vs. ~ 3) suggest that Au nanoclusters (< 1.0 nm) inside the TS-1 nanopores are gold active sites for propylene epoxidation. The catalytic activities for Au/un TS-1 at the early stage of reaction (ie, PO rate of Au/un TS-1 Cs is ~ 2.0 gPO h-1 kgTS-1-1) indicate that Au nanoclusters (< 1.0 nm) on the external surfaces of TS-1 also play an important role. Therefore, Au nanoclusters (< 1.0 nm) on the external surfaces of TS-1 and inside the TS-1 nanoporous channels are the dominant gold active sites for gas-phase epoxidation of propylene. It is hoped that better PO performance could be achieved by controlling the size of Au clusters smaller than 1.0 nm over Au/TS-1. Supporting Information Schematic diagram of the reaction apparatus; XRD patterns and UV-vis spectra of Au/TS-1, Au/un TS-1 and Au/S-1/TS-1 catalysts; Ti 2p photoelectron spectra of Au/TS-1 Na and Au/TS-1 Cs; Performances for propylene epoxidation of catalysts spent at 200 °C; HADDF image of Au/TS-1 and the corresponding EDS analysis of Au-L and Au-M; HRTEM image of spent Au/TS-1 Cs and (b) the corresponding Au

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particle size distributions; PO rate vs. time on stream in hours for reused catalyst; HRTEM images of Au/un TS-1 Cs kept for long time and calcined at 200 °C, and the corresponding PO rates vs. time-on-stream in the first 2 hours. Notes The authors declare no competing financial interest. Acknowledgments This work is supported by the National Natural Science Foundation of China (No.21276127) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERRENCES (1) Qi, C. The Production of Propylene Oxide over Nanometer Au Catalysts in the Presence of H2 and O2. Gold Bull. 2008, 41, 224-234. (2) Ainsworth, S. J. Market for Polypropylene is Strengthening as Economy Improves. Chem. Eng. News 1992, 70, 9-11. (3) Huang, J.; Takei, T.; Ohashi, H.; Haruta, M. Propene Epoxidation with Oxygen over Gold Clusters: Role of Basic Salts and Hydroxides of Alkalis. Appl. Catal. A Gen. 2012, 435-436, 115-122. (4) Hayashi, T.; Tanaka, K.; Haruta, M. Selective Vapor-Phase Epoxidation of Propylene over Au/TiO2 Catalysts in the Presence of Oxygen and Hydrogen. J. Catal. 1998, 178, 566-575. (5) Nguyen, V. H.; Chan, H. Y.; Wu, J. C. S.; Bai, H. Direct Gas-Phase Photocatalytic Epoxidation of Propylene with Molecular Oxygen by Photocatalysts. Chem. Eng. J.

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Page 24 of 35

Table 1 Summary of prepared supports BET surface ares a

Pore volume b

Ti content c

(m2g-1)

(cm3g-1)

(wt%)

TS-1

425

0.258

0.830

un TS-1

15

0.168

0.571

S-1/TS-1

406

0.273

0.145

supports

a

Determined by BET.

b

Determined at P/P0=0.995.

c

Determined by ICP-AES.

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Table 2 Performances for gas-phase epoxidation of propylene

catalysts

Au loading (wt%)

PO rate b

a

(gPO h

-1

-1

(gPO h

-1

PO H2 C 3 H6 selectivity efficiency conversion

kgcat )

kgTS-1-1)

(%)

(%)

(%)

Au/TS-1 Na

0.058

125

125

95

33

3.6

Au/TS-1 Cs

0.314

330

330

87

10

10.5

0.025

30

—

93

23

0.9

0.092

58

—

95

27

1.7

0.043

45

94

98

34

1.3

0.149

122

255

98

20

3.5

0.156

130

270

97

21

3.7

Au/un TS-1 Na Au/un TS-1 Cs Au/S-1/TS-1 Na 10 h Au/S-1/TS-1 Cs 10 h Au/S-1/TS-1 Cs 20 h a

Determined by ICP-AES.

b

Rates are the average value of the first 1-4 h at 200 °C for Au/TS-1 and Au/S-1/TS-1.

Rates are the value reported after 30 h of steady-state activity at 200 °C for Au/un TS-1. Reaction conditions: C3H6：H2：O2：N2=3.5：3.5：3.5：24.5 mL min-1，space velocity=14000 mL h-1 gcat-1.

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Page 26 of 35

Table 3 Element contents on the surface of Au/TS-1 catalyst

a

catalysts

Si (wt%) a

Ti (wt%)

Au (wt%)

Na/Cs (wt%)

Au/TS-1 Na

27.6±1.3

0.3

< 0.1

0.3

Au/TS-1 Cs

26.9±1.4

0.3

< 0.1

0.4

Determined by XPS analysis.

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Figure 1 (a) XRD patterns, (b) UV-vis spectra, (c) FTIR spectra and (d) N2 adsorption-desorption isotherms of TS-1, un TS-1 and S-1/TS-1

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Figure 2 FESEM images and EDS analysis of (a) TS-1, (b) un TS-1 and (c) S-1/TS-1

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Figure 3 TEM images of (a) Au/TS-1 Na and (b) Au/TS-1 Cs (a’ and b’ are the corresponding partial enlargement images)

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Figure 4 Diagrams of the changes of Au loading and Au particle sizes on Au/TS-1 Na and Au/TS-1 Cs

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Figure 5 TEM image of S-1/TS-1

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Figure 6 Diagrams of the changes of Au loading on Au/S-1/TS-1 Na and Au/S-1/TS-1 Cs

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Figure 7 PO rates vs. time-on-stream at the early stage for Au/un TS-1 Na and Au/un TS-1 Cs spent at 200 °C, respectively.

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Figure 8 Diagram of the reaction over Au nanoclusters (< 1.0 nm) on the external surface of un TS-1

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TOC Graphic

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Confirmation of Gold Active Sites on Titanium-Silicalite-1-Supported

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